<%@ page language="java" contentType="text/html; charset=UTF-8"
    pageEncoding="UTF-8"%>
<%@ taglib uri="http://java.sun.com/jsp/jstl/core" prefix="c" %>    
<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title>Insert title here</title>
</head>
<body>
<%
	// pageContext.setAttribute("username", "admin");
%>
<!-- 演示c:if标签 -->
<c:if test="${not empty username}">
	存在用户名
</c:if>

<c:if test="${empty username }">
	不存在用户名
</c:if>
</body>
</html>